DIYbanter - View Single Post - Trying to figure out heat...does this make sense?

wrote:

So for the past few years, I would keep the thero at 68 during the day
while I am at work, then to 70 when I am home.

With the increase in oil cost and more inefficiency in my heating
system (I am working on it), I am needing to put the thermo to 72
degrees when I am home to be comfortable.

Here is a formula:

Old way:

10 hours @ 68 degrees = 680
14 hours @ 70 degrees = 980

Total of hours x degrees = 1660

New way:

10 hours @ 65 degrees = 650
14 hours @ 72 degrees = 1008

Total of hours x degrees = 1658

Does heat work that way, or does it take more juice to get to 72
degrees instead of 70?
Feel free to tell my how foolish my heat calculation is.

Bluesman

Your looking at it in the right direction, but your method of
calculation isn't correct.

The first thing to realize is that heat always runs "downhill" from hot
materials to cold ones. and the way to look at it is that the amount of
"juice" as you put it, is basically delta-T (the difference in inside
and outsided temperatures) times a factor for whatever "thermal
resistance" the heat flows through, in your case, that's the insulation
of the walls, roof and windows of your dwelling. Winds outside come into
it too, as a second order effect.

And yes, all other things being equal it does take "more juice" the get
to 72 than to 70, and more to keep it at 72 than at 70, when the outside
temperature is below 70.

The simplest and most efficient way to do what you are talking about is
to set the heat as low as you can when you're not home, as long as you
don't run into the possibility of freezing plumbing, and then turn it up
to 72 or whatever you want when you return. That's providing you don't
mind whatever waiting time your heating system takes to raise the
temperature to where you get comfortable. If you can't tolerate the
wait, then set your "not home" temperature a little higher.

You might also want to consider going a little higher tech and using a
timed thermostat set to raise the temperature a bit before your expected
return, or use some of the whiz-bang gadgetry that lets you control
stuff like that with a telephone call.

Happy Holidays,

Jeff

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"As long as there are final exams, there will be prayer in public
schools"